A Multifunctional Na<i>_x</i>Bi/NaCl Flexible Interface Layer for Solid‐State Na Metal Batteries

Abstract Na 3 Zr 2 Si 2 PO 12 (NZSP) electrolyte with excellent air stability and acceptable ionic conductivity demonstrates a significant potential for application in solid‐state Na metal batteries. However, the poor interfacial contact between NZSP and Na anode has severely hindered its practical application. Herein, a BiCl 3 /polytetrafluoroethylene flexible interface layer is constructed between NZSP and Na metal anode. BiCl 3 can react with Na metal to form a multifunctional Na x Bi/NaCl‐rich flexible interface layer during the cycling process, which can effectively enhance the interfacial contact, avoid the formation of voids and reduce the interfacial resistance. Na x Bi with low energy barrier can provide fast Na + diffusion path, and NaCl with a wide band gap can inhibit electron injection and suppress Na dendrite growth. Owing to the multifunctional Na x Bi/NaCl flexible interface layer, the Na/BiCl 3 @NZSP/Na symmetric cell exhibits a reduced interfacial resistance from 1252.1 to 67.6 Ω and achieves a high critical current density of 2 mA cm⁻ 2 at 25 °C. At 0.1 mA cm⁻ 2 /0.1 mAh cm⁻ 2 and 0.3 mA cm⁻ 2 /0.3 mAh cm⁻ 2 , the symmetric cell can stably cycle for 3000 and 1100 h. Additionally, the Na 3 V 2 (PO 4 ) 3 /BiCl 3 @NZSP/Na full cell can retain 96.7% of initial capacity (≈111 mAh g −1 ) after 300 cycles at 0.5 C and excellent rate capability.